In Bierce's "Haïta the Shepherd", which appeared in the collection Can Such Things Be?, Hastur is more benevolent than he would later appear in August Derleth's mythos stories. Another story in the same collection ("An Inhabitant of Carcosa") referred to the place "Carcosa" and a person "Hali", names which later authors were to associate with Hastur.

In Chambers' The King in Yellow (1895), a collection some of which are horror stories, Hastur is the name of a potentially supernatural character (in "The Demoiselle D'Ys"), a place (in "The Repairer of Reputations"), and mentioned without explanation in "The Yellow Sign". The latter two stories also mentioned Carcosa, Hali, Aldebaran, and the Hyades, along with a "Yellow Sign" and a play called The King in Yellow.

H. P. Lovecraft read Chambers' book in early 1927 and was so enchanted by it that he added elements of it to his own creations. There are two places in Lovecraft's own writings in which Hastur is mentioned:

I found myself faced by names and terms that I had heard elsewhere in the most hideous of connections—Yuggoth, Great Cthulhu, Tsathoggua, Yog-Sothoth, R'lyeh, Nyarlathotep, Azathoth, Hastur, Yian, Leng, the Lake of Hali, Bethmoora, the Yellow Sign, L’mur-Kathulos, Bran and the Magnum Innominandum—and was drawn back through nameless aeons and inconceivable dimensions to worlds of elder, outer entity at which the crazed author of the Necronomicon had only guessed in the vaguest way.... There is a whole secret cult of evil men (a man of your mystical erudition will understand me when I link them with Hastur and the Yellow Sign) devoted to the purpose of tracking them down and injuring them on behalf of the monstrous powers from other dimensions. —H. P. Lovecraft, "The Whisperer in Darkness"

It is unclear from this quote if Lovecraft's Hastur is a person, a place, an object (such as the Yellow Sign), or a deity (this ambiguity is recurrent in Lovecraft's descriptions of the mythic entities).

"... after stumbling queerly upon the hellish and forbidden book of horrors the two learn, among other hideous things which no sane mortal should know, that this talisman is indeed the nameless Yellow Sign handed down from the accursed cult of Hastur—from primordial Carcosa, whereof the volume treats..."

"...We spoke of Hastur and of Cassilda..."

So, judging from these two quotes, it is quite possible that H. P. Lovecraft not only recognized Hastur as one of the mythos gods, but even made him so recalling Chambers' book.

Derleth also developed Hastur into a Great Old One, spawn of Yog-Sothoth, the half-brother of Cthulhu, and possibly the Magnum Innominandum. In this incarnation, Hastur has several Avatars:

Anders Fager's "Collected Swedish Cults" features a Stockholm-based coterie known as "The Carcosa Foundation" that worships Hastur.

Hastur is amorphous, but he is said to appear as a vast, vaguely octopoid being, similar to his half-niece Cthylla.